Wednesday, March 23, 2011

EIGRP Next Hop, RIPv2 Next Hop, and OSPF Forward Address

In IGRP, if router X sends an update to router A with a destination network number of N, router A's next hop for packets to N will be X. In EIGRP, router X can send an update to router A with a destination network number of N and a next hop field of Y. This is useful, say, in a scenario where X and Y are running RIP and X is redistributing routes from RIP to IGRP. When X sends an update to its neighbors on a shared network, X can tell them to send traffic for network N directly to Y and not to X. This saves X from having to accept traffic on a shared network and then reroute it to Y.OReilly - IP Routing - Chapter 4 - Enhanced Interior Gateway Routing Protocol (EIGRP)http://oreilly.com/catalog/iprouting/chapter/ch04.html

My way of studying networking is always setup simulation labs to see things in action and prove them really work as claimed.

Firstly, let's have a look on the packet formats for EIGRP IP Internal and EIGRP IP External routes.

EIGRP IP Internal Route Packet Format

EIGRP IP External Route Packet Format

Take note upon the Next Hop field in the packet formats, we are going to see how it really works. :-)
Whether it works as claimed in the OReilly book. *grin*

The figure above shows the network setup used for our demonstration purpose.
RT1, RT2, and RT3 reside on the same subnet - 10.10.10.0/24.
RT1 and RT2 will be setup in sequence as RIPv2, OSPF, and EIGRP neighbors...
RT2 is configured with a static route to 172.16.1.0/24 via RT3.
RT2 is redistributing the static route into RIPv2, OSPF, and EIGRP respectively.
Theoretically, RT2 should advertise the external route to RT1 with the next hop of RT3 in order to eliminate the extra hop of routing via RT2, which is inefficient.

OSPF also works efficiently as like RIPv2.
OSPF utilizes the Forwarding Address field in the Type-5 AS-External LSA which is similar to the Next Hop field in the RIPv2 Update packets.
Below shows the OSPF LSU packet generated by RT2 to RT1 for the sake of completeness. :-)

hmm... the next hop is not RT3 as like RIPv2 and OSPF.
ICMP redirect packets were generated by RT2 every time as a packet originated from RT1 to 172.16.1.0/24.
The ICMP layer of RT2 is trying to solve the inefficiency problem in EIGRP, but RT1 is too stubborn to listen to the suggestion / recommendation by RT2. :-)

Hi Amit, should be a basic stuff for RIP and not that I know it is IOS specific. Please send some show version, show running and show ip route to yapchinhoong@hotmail.com if you require another pair of eyes to look into it. Thanks. :)